EdmundYeung99's answer is as much of an answer as exists in the film, and is in every sense the 'correct' answer. However ...
Having just re-watched the film, I have just constructed a non-canon fan-theory that can work if you make one tiny change to the film that otherwise has no effect, and gives a much more satisfying answer....
Time Travel's Zero-Point is 1981.
In the film, Agent Doe takes John to the Temporal Bureau, and leaves him there in August 1985. Shortly after (the next day?) John is given his first Field Kit and
becomes Agent Doe.
That date, August 1985, doesn't tie into anything, AFAICS. There are no references to any of John's early missions at the TB, nor any other reference to when he became an Agent.
Supposing you make a single character change to the film: change the last digit on the clock to a '1'. Now John becomes Agent Doe in the same year that Time Travel is invented.
From there it isn't much of a leap to suggest that:
The very existence of (Jane/John/Agent Doe/Fizzle Bomber) is the fundamental source of Time Travel. That (continuing the theme of John's circular life) Time Travel exists solely because this closed temporal loop exists ... which of course only exists because Time Travel exists.
That explains the 1981 start point. To get the 53-year limit ...
one could assert/guess that the Fizzle Bomber is maybe 53 years old (in terms of his personal linear TimeLine) when he dies. It doesn't seem to far off visually. And have a nice feel to it.
Obviously all of this is completely made-up and does require a tiny change to the film. But I think it's rather nice :)
